During the past decade our understanding of the dynamic integrative capacity of astrocytes, a sub-type of glial cell of the CNS, has dramatically increased. While once thought to play only supportive roles, recent data suggest that neuronal activity raises astrocytic calcium levels, which in turn cause a feedback release of chemical transmitter from astrocytes to modulate neighboring neurons. We will use acutely isolated hippocampal slice preparations to test the hypothesis that calcium elevations in astrocytes cause neuromodulatory functions resulting from the calcium-dependent release of glutamate from astrocytes. To achieve this experimental objective we will perform integrated studies in which we monitor internal calcium using confocal microscopy, manipulate astrocytic calcium levels using focal flash photolysis of caged compounds, and record synaptic transmission using patch clamp recordings. In order to unambiguously identify astrocytes, we will use transgenic mice in which green fluorescent protein (GFP) is expressed selectively in astrocytes. We will determine whether: 1) hippocampal astrocytes exhibit functional compartmentalization of calcium signaling within their processes; 2) waves of elevated calcium can propagate between astrocytes in hippocampal slices; 3) the calcium-dependent release of neurotransmitters from astrocytes regulates internal calcium levels and the excitability of neighboring neurons and astrocytes; 4) the calcium-dependent release of glutamate from astrocytes dynamically controls synaptic transmission. By performing these studies we will obtain new insights into the roles of astrocytes in integration in the CNS. Since astrocytes can integrate neuronal inputs and can release glutamate in response to elevated internal calcium, the demonstration of a neuromodulatory capacity for astrocytes has the potential to significantly change the way we view computation in the nervous system.